Documents which exist in electronic form can be copied as often as desired without loss of quality. For that reason, reliable methods must be employed to prevent such documents from being freely disseminated without control, in order to protect the rights of the intellectual property owner.
Due to the rapid growth of the Internet and the capability it provides for digitally disseminating documents, there is an increased requirement to protect against the illegal dissemination of documents and, thus, to protect a copyright owner from pirated copies.
For this reason, large firms, such as IBM, NEC and Microsoft, and smaller firms as well, such as Digimarc (see Funkschau 17/97; p. 21) and research institutes, such as the Fraunhofer Company IGD and the GMD Darmstadt, have worked on embedding so-called digital watermarks in documents. In methods having such a basis, information identifying the copyright owner is introduced as invisible information into the documents to be protected. It is hidden in the document in such a way that no outsider can discover it. Only the owner himself can make the watermark visible by using his secret key and, therefore, in the case of a legal dispute, for example, prove that he is actually the owner.
There can be different kinds of inserted digital watermarks and, in this context, each can depend on the particular type of document (e.g., postscript, JPEG, MPEG-1).
Thus, for example, Schneider, M. et al., in the essay: “ROBUST CONTENT BASED DIGITAL SIGNATURE FOR IMAGE AUTHENTICATION” in Proc. Intl. Conference on Image Processing (ICIP) New York, U.S., IEEE, 1996, pp. 227-230, describe a method for embedding digital signatures as hidden signatures into the useful data for verifying the authenticity of data, i.e., proving that the data have been manipulated with, in that signatures are extracted using hash functions, and the result is combined with a private key, so that, altogether, a signature is formed which contains characteristics of the original work, as well as the identity of the author.
As described in this publication, such a signature can be transmitted concurrently with the data of the original work or also be hidden therein in such a way that it also serves the purpose of a watermark. Also, as described in this publication, the digital watermark can additionally be provided with an authentic time stamp.
U.S. Pat. No. 5,499,294 also describes generating a digital signature, which is associated with an original image and which encompasses both a hash value as well as a private key. However, this signature is not used in a watermark.
U.S. Pat. No. 5,809,160 describes a method for embedding signature information in original data as watermarks, however, without mentioning a hash function.
In addition, the abstracts of German Patent Application No. 196 15 301 and European Patent 0 845 758 A3 describe embedding a digital signature in data that need to be able to be authenticated, in each case a key or a secret key being combined with an extract of the data to form an embedded signature.
Digital watermarks make it possible for a copyright owner to prove that an illegally disseminated document is his or her intellectual property. However, digital watermarks do not make it possible to determine who the originator of the illegal dissemination is, nor to prove that such a person did in fact illegally disseminate the document. This is because, in contrast to electronic fingerprints, digital watermarks do not contain any indication of an authorized recipient of a copy of the document. If such a recipient himself wants to further disseminate the document and appear to be the originator, he can likewise provide the document with his digital watermark. This can lead to the paradoxical situation in a legal dispute that both opposing parties can verify their watermark in the document at issue and each one can accuse the other of the unauthorized copy.
In such a case, the court can only pass correct judgment when the true originator can also prove a document that does not have either watermark or that only has his watermark, and not that of the opposing party. However, it can be impossible to provide such a proof, especially when working with very voluminous documents that are only available in one copy provided with a digital watermark, on one publicly accessible server.